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Date:   Thu, 29 Jul 2021 08:55:46 +0200
From:   Pablo Neira Ayuso <pablo@netfilter.org>
To:     Tom Yan <tom.ty89@gmail.com>
Cc:     netfilter-devel@vger.kernel.org
Subject: Re: [PATCH nft 2/3] netlink_linearize: incorrect netlink bytecode
 with binary operation and flags
Message-ID: <20210729065546.GA15962@salvia>
References: <20210727153741.14406-1-pablo@netfilter.org>
 <20210727153741.14406-2-pablo@netfilter.org>
 <CAGnHSE=e-iYaz3KOMBq1JSVRd0HTL=TKQ_HHMadfyd2Nr8__yg@mail.gmail.com>
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On Thu, Jul 29, 2021 at 10:57:35AM +0800, Tom Yan wrote:
> On Wed, 28 Jul 2021 at 05:05, Pablo Neira Ayuso <pablo@netfilter.org> wrote:
[...]
> > A quick summary:
> >
> > - If you want an exact match:
> >
> > tcp flags == fin,syn,ack
> >
> > - If you want to check that those three bits are set on (regardless
> >   the remaining bits):
> >
> > tcp flags fin,syn,ack / fin,syn,ack
> >
> > - If you want to check that any of these three bits is set on:
> >
> > tcp flags fin,syn,ack
> 
> This is exactly what I find absurd btw. IMHO it's much better if the
> latter just means `tcp flags == (fin | syn | ack)`.

Look at this from a different angle, ie. ct state

        ct state new,established

ct state also has a bitmask datatype, and people are not expecting
here to match to new AND established.

> I'd rather we keep `tcp flags & (fin | syn | ack) != 0` and so
> "unsimplified" or accept something like `tcp flags { fin / fin, syn
> / syn, ack / ack }`

The curly brace notation implies the use of sets. Sets only allow for
exact matches, therefore

tcp flags { fin, syn, ack}

is actually making exact matches on the tcp flags.